The present invention relates generally to testing of electronic communications devices and more particularly to loopback testing of an electronic communications device.
Testing of certain electronic communications devices, such as a high speed transceiver, is a problem for production. The data rate of, for example, a fiber channel transceiver chip (or similar device) is too fast for direct testing by means of commercially available production testers. For this reason, a xe2x80x9cloopbackxe2x80x9d function is added to a transceiver.
Typically, the loopback function connects the transmit output of the transceiver back around to the receiver input of the transceiver internally (i.e., by bypassing the driver and receiver buffers). In this manner, data can be sent to the transmit function and back to determine whether the receive function has received the same data as was transmitted. If a string of random or pseudo random data is sent, this testing mechanism further provides a functional test of most logic except for the high speed I/O pins. If the high speed I/O pins are connected to each other to form a loopback external to the device, then more of the device is tested.
One problem with loopback, however, is that it does not mimic actual system operation very well. Loopback forces the transmit and receive functions to be synchronous and to operate at exactly the same baud rate. In an actual system, the receive data stream is asynchronous to the transmit stream and at a slightly different baud rate. Thus, if the device under test has a problem with handling asynchronous or frequency offset data, loopback won""t detect it. As an example, a device or design flaw in a device such as an SN75FC10001-gigabit fibre channel transceiver chip, a device made and sold by the assignee, Texas Instruments Incorporated, and described in datasheet SLLS255B dated August 1996, revised December 1996, herein incorporated by reference in it""s entirety, that causes unwanted interactions between the transmit and receive functions will not be detected in a loopback test.
The present invention is an improved loopback testing method and system for an electronic communications device. The electronic communications device includes a receive function and a transmit function. A mechanism for asserting a frequency offset on the transmit function to perform tests of the receive function when the electronic communications device is in a testing mode is provided.
In one aspect of the present invention, the asserting means includes a transmit phase interpolator.